1. Field of the Invention
The present invention relates to a semiconductor laser apparatus, and more particularly to an external resonator type semiconductor laser apparatus having a reflecting film provided outside a resonator.
2. Description of Related Art
Generally, the characteristic of laser light output from a semiconductor laser depends on its operating temperature. More specifically, the oscillation wavelength and output amplitude of the laser light depends on the temperature. When the ambient temperature changes in a semiconductor laser oscillating at a predetermined wavelength, the oscillation wavelength does not vary proportionally to the temperature change, but becomes longer stepwise following the temperature rise.
In order to reduce such dependency on temperature as referred to above, there has been proposed a distributed feedback type semiconductor laser, or a distributed Bragg reflector type semiconductor laser, having an active layer to produce a diffraction grating effect. Such semiconductor laser, which has a diffraction grating provided directly at the active layer within a laser device, is complicated to manufacture and is difficult to use with some semiconductor materials.
In a typical semiconductor laser, for example, a double hetero-structure type semiconductor laser having an approximately rectilinear active layer and a Fabry-Perot resonator, it has been proposed to add an external resonator type semiconductor laser apparatus having a reflector provided outside the resonator (referring to West German Patent Laid-Open No. DE3442188A1, etc.). Such semiconductor laser apparatus is provided with a reflector outside the resonator opposite to a light emitting facet of the semiconductor laser. In this case, the external reflector is a photodiode coated with a metal film and is located at a distance L from the light emitting facet of the semiconductor laser which is an integral multiple of 1/2 of the oscillation wavelength [L=.lambda..sub.e (me+1/2).multidot.1/2]. Such construction reduces the temperature dependency of the oscillation wavelength and controls the change or fluctuation in the oscillation wavelength as a result of the temperature change because of the cooperative effect of the internal resonance mode and external resonance mode.
It is also known to provide a facet of a photodetector formed by a cleavage, instead of a photodiode with a dielectric film in place of the metal film (referring to U.S. Pat. No. 4,803,695).
However, there are two problems regarding the above. One problem is the balance between the photodetecting properties of a photosensor and the efficiency of the external resonance mode. It is better, when attempting to increase the efficiency of the external resonance mode to increase the reflectivity of the external reflector. However, when the reflectivity is raised, the photodetecting sensitivity of the photodetector, such as a photodiode, is decreased.
In order to stably drive the semiconductor laser, an output of the photosensor is used for providing feedback-control with respect to a driving current for the semiconductor laser. With this arrangement, when the output of the photosensor is small, the change in the optical output cannot be discriminated from noise, or a time lag is caused in the feedback-control, thereby hindering stabilized operation of the semiconductor laser. Also, the reflected light, when intense, is liable to interfere with a far-field pattern which is utilized for communication or information reading of another laser light. As a countermeasure for the above, the reflecting film is provided at a part of the cleavage facet at the photodetector as disclosed in the aforesaid U.S. Pat. No. 4,803,695. However, in this arrangement, it is difficult to position the reflecting film and adjust its angle in order to accurately oppose the same to the light emitting facet of the semiconductor laser.
Another problem with the aforesaid prior art devices is that a thin film is utilized as the reflecting film and, accordingly, the optical characteristic of a support for the reflecting film affects the external resonance mode. In other words in the aforesaid U.S. Pat. No. 4,803,695, wherein a dielectric film is provided at the cleavage facet of a semiconductor photodetector, the reflectivity of the dielectric film is obtained by the cooperative effect of the reflection at the surface of the thin film and the reflection at the interface between the film and the cleavage facet. Further, the reflection within the crystal of the photodetector, is disclosed in Japanese Patent Application Laid-Open No. 229889/1988. Accordingly, in the arrangement of the crystalline structure and external semiconductor chip of GaAs or the like used as the photodetector, it is an important factor to specify the external resonance mode, similarly to positioning of the photodetector with respect to the semiconductor laser. Hence, precise and complicated manufacturing work is required.